(a) Field of the Invention
The present invention relates to an LED light emitting device and a driving method. More particularly, the present invention relates to an LED light emitting device applying a pulse width modulation method, and a driving method thereof.
(b) Description of the Related Art
An LED light emitting device includes an LED channel consisting of a plurality of LED elements that are arranged in series, and a constant current source for controlling a current flowing through the LED channel. The constant current source includes a sink current source, and the sink current source controls a flowing current through each of the LED channel by using an operational amplifier to be constant.
There are an analog method and a pulse width modulation method to control brightness of the LED element. The analog method controls the size of a continuously flowing current through the LED element. The pulse width modulation method controls a pulse width of a current that has a constant size and discontinuously flows through the LED element. The analog method has a change of color coordinate when a current flowing through the LED element is small, but since the pulse width modulation method has no change of color coordinate, the pulse width modulation method is mainly used.
FIG. 1 is a waveform diagram showing a pulse width modulation signal, a channel current flowing through an LED channel, and an output voltage at an operational amplifier. In FIG. 1, (b) is an enlarged illustration of a section in which the pulse width modulation signal transits from a high level to a low level in the waveform diagram of (a).
Referring to FIG. 1, the pulse width modulation PWM signal is a high pulse signal having a certain period. The operational amplifier outputs a high level output voltage Va for a period in which the pulse width modulation PWM signal is a low level. A channel current ILED flows through the LED channel for the period in which the output voltage Va at the operational amplifier is a high level for a predetermined time.
However, as shown in (b), when the pulse width modulation PWM signal descends from a high level to a low level, the output voltage Va at the operational amplifier does not rise according to the pulse width modulation PWM signal. It has the predetermined delay time. This is because of a power-on delay of the operational amplifier. Due to this, the predetermined delay is generated in the channel current ILED.
That is, there is a problem that the output voltage Va at the operational amplifier and a slew rate of the channel current ILED are degraded. Moreover, in order to solve the power-on delay of the operational amplifier, a large current can be supplied to the operational amplifier. But this generates unnecessary power consumption.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.